1. Field of the Invention
The present invention relates to a method for controlling the printing impact power in an impact type dot printer, and more particularly to a method for controlling the printing impact power in an impact type dot printer to change the printing impact power of printing stylus in a high density imprint function from the printing impact in an normal density imprint function.
2. Prior Art
In recent years, an impact type dot printer has been of wide use in the various printing devices in order to increase the printing speed and to simplify the printing control.
In ordinary cases, the apparatus of this kind is equipped with a printing function of such a normal imprinting density, as shown in FIG. 1, that the dot gaps are put in the contact state, and with a printing function of such a high imprinting density, as shown in FIG. 2, that each of the dots is put in the overlap state.
This high density imprint function is characterized in that the overlap imprints enable easy reading in comparison with the characters printed through normal imprint density since the dot formed on the papers through the overlap imprints are imprinted in the overlapping joined relation. When the characters are required to be printed in good appearance, the high density imprint function is highly recommended. On the contrary, when the characters are not needed to be of a good appearance and the printing operation must be finished in a short time, the characters are imprinted with the normal density imprint function.
The selection of said normal or high density printing mode is normally controlled by the use of normal/high density selecting commands which are included in printing data supplied from a host computer to the printer. Therefore, such commands can perform the desired density selection with respect to particular characters or particularly ranged character rows in the printing data or with respect to particular articles as a whole. It is also possible that said selecting commands are not included in the printing data. In such a case, the printer will be manually controlled by an operator through a keyboard or control panel which is adapted to control the printing operation of the printer. Normally, such a manual operation can perform only a collective selection from the normal density printing to the high density printing or vice versa with respect to a whole article to be printed.
The prior art printer is adapted to perform the changing operation of character generator and paper feed pitch when said selecting command is inputted into the printer. In other words, the prior art printer must supply stylus driving signals to the printing head, based on the inputted printing data. Thus, the printer will include character generators which are adapted to generate dot pattern signals associated with the printing data. For the aforementioned selection of the normal/high density printings, the printer must have two separate character generators, that is, a normal density character generator and a high density character generator. On receiving said selecting commands, necessary dot pattern signals are applied to the printing head from one of these character generators. Since the paper feed pitch in the normal density printing mode is different from that in the high density printing mode, the selecting commands should change the paper feed pitch in the printer to an optimum value for the corresponding printing mode.
In such a manner, the prior art printer can perform the selection of the printing density as shown in FIGS. 1 and 2 only after the change of the character generator and papaer feed pitch has been done, in accordance with the density selecting command which is inputted to the printer through the host computer or keyboard.
However, in the printing function of such a device in the prior art, the high density imprint damages the printing papers as a result of overlapped impacts by the printing stylus in plurality to the same spot on the printing papers, since the impact power to the printing paper is fixingly determined with the basis of printing impact power at the normal density imprint function.
In the prior art, therefore, a sheet of printing paper can be torn if the printing data including two kinds of printing density data, that is, normal and high dot density data different from each other are used on printing only by the change of the character generator and paper feed pitch.
A device capable of changing the printing pressure or stylus driving force depending on different printing patters has been proposed,for example, by Japanese Laid-Open Patent Application No. 57-64570.
However, such a device is not adapted to change the impact power of dot printing with respect to the normal and high density printing modes, but to change the stylus driving force depending on different printing patterns. If a simple character as "I" or "L" and a complicated character as "M" or "W" are printed by the use of the same stylus driving force, the former appears to be thin but the latter appears to be thick. To overcome such a difference in the printed characters, the prior art device is adapted to change the stylus driving force. Furthermore, the prior art device is adapted to reduce the stylus driving force to prevent the printing paper from easily being torn when a ruled line or underline is printed thereon. This is not because the dot density is increased, but because the ruled line or underline is in the form of a continuous row of dots extending through an increased distance to weaken the printing paper along the ruled line or underline. This fact is essentially different from such a problem that the printing paper is weakened at overlapped dots which are printed thereon in the high density printing mode.
The prior art device also is required to have a counter or other circuit for counting the number of dots in a printed pattern. This will make the device more complicated and expensive and also results in increase of the necessary printing period. In other words, the prior art device must judge the dot density in the corresponding printing data to discriminate whether a character or dot pattern therein is in the form of or "M". This requires a complicated circuit and an increased processing time period.
There has not been known a method of changing the stylus driving force on selection of the printing mode from the normal density to the high density or vice versa to provide printed characters having an improved quality without tearing of the printing paper, particularly in the high density printing mode.